I have tried to use the new operators of D2 and I have found several problems. This small program shows two of those problems (some of my problems can be caused by my improper usage, I'm trying to tell apart the operator overloading bugs from my improper usage cases).

Do you know if/how this program can be fixed?


struct Foo {
    int x;

    Foo opUnary(string op:"++")() {
        this.x++;
        return this;
    }

    uint opCast(T:uint)() {
        return this.x;
    }
}

void main() {
    Foo f = Foo(5);
    f++; // line 16
    auto a = new int[f]; // line 17
}

Errors:
temp2.d(16): Error: var has no effect in expression (__tmp1)
temp2.d(17): Error: cannot implicitly convert expression (f) of type Foo to uint

Note: the line 17 works if I write it this way, but it's not nice:
auto a = new int[cast(uint)f];

Bye,
bearophile

On Sun, Mar 14, 2010 at 14:14, bearophile <bearophileHUGS@lycos.com> wrote:

> I have tried to use the new operators of D2 and I have found several
> problems. This small program shows two of those problems (some of my
> problems can be caused by my improper usage, I'm trying to tell apart the
> operator overloading bugs from my improper usage cases).
> void main() {
>    Foo f = Foo(5);
>    f++; // line 16
>    auto a = new int[f]; // line 17
> }
>
> Errors:
> temp2.d(16): Error: var has no effect in expression (__tmp1)
>

Try ++f, it works. I guess ++f is rewritten f.opUnary!"++"(), but f++ must have create a temporary variable.



> temp2.d(17): Error: cannot implicitly convert expression (f) of type Foo to
> uint
>
> Note: the line 17 works if I write it this way, but it's not nice:
> auto a = new int[cast(uint)f];
>
>
Note the error message: cannot *implicitly* convert expresion (f).
opCast defines the explicit cast operation, it has no effect on implicit
casting. Having an opImplicitCast(T) would be nice, I agree.

For now, this also does not work:

void bar(int i) {}

(in main:)
   bar(f); // Error: cannot implicitly convert expresion (f) of type Foo to
int.

That's sad, I'd like to have std.variant.Variant defining an opCast. That way, you could do:

void foo(int i) {}
void bar(string s) {}

Variant v;
v = 3;
foo(v);       // try to cast v to int implicitly: works, move along.
v = "abc";
bar(v);       // implicit cast to string, works.


  Philippe

On 14/03/10 13:14, bearophile wrote:
> I have tried to use the new operators of D2 and I have found several problems. This small program shows two of those problems (some of my problems can be caused by my improper usage, I'm trying to tell apart the operator overloading bugs from my improper usage cases).
>
> Do you know if/how this program can be fixed?
>
>
> struct Foo {
>      int x;
>
>      Foo opUnary(string op:"++")() {
>          this.x++;
>          return this;
>      }
>
>      uint opCast(T:uint)() {
>          return this.x;
>      }
> }
>
> void main() {
>      Foo f = Foo(5);
>      f++; // line 16
>      auto a = new int[f]; // line 17
> }
>
> Errors:
> temp2.d(16): Error: var has no effect in expression (__tmp1)
> temp2.d(17): Error: cannot implicitly convert expression (f) of type Foo to uint
>
> Note: the line 17 works if I write it this way, but it's not nice:
> auto a = new int[cast(uint)f];
>
> Bye,
> bearophile

I don't know about your first problem, but the latter is because opCast is only for explicit casts. I think implicit casts can be emulated with alias this, eg:
----
alias x this;
----
I'm not sure though as I haven't tested it. This also means that anything that isn't defined in Foo will be forwarded to x though, which might not be what you want.

Philippe Sigaud:
> Try ++f, it works.

I know, but that's not what I wanted.
Thank you to you and Robert Clipsham for your answers, now I present again the same things, plus few others, in the main D newsgroup, to see what they think.

Bye,
bearophile